The present invention is directed to lead-acid batteries, and particularly to batteries having lightweight, high energy electrochemical cells in which the electrode plates are formed of a foil substrate covered by a continuous, non-corrosive, conductive coating.
Small lead-acid electrochemical cells used for example, as the power source for small electronics, flashlights, and the like have previously been made of thin films of lead coated on each side with an electrochemical paste (positive or negative). Cells are completed by stacking or winding alternating positive and negative plates with separators therebetween.
Lead has been predominately used as the plate material in such batteries for a long period of time. While lead is not particularly a good conductor of electricity, it is inherently resistant to the corrosive effect of the electrolytic acids. Other, more conductive metals are either too expensive to be used as the electrode for lead-acid batteries, or else they are quickly corroded during the charging action by the electrolytic acids. Therefore lead has remained as the predominant material. Lead is also very heavy, and in applications where weight or increased conductivity is a factor, other alternatives have long been sought.
In previous attempts, one approach has been to plate lead onto other more conductive metals or metal alloys such as aluminum and copper. Copper is sixteen times as conductive as lead and weighs only about 70% as much. Aluminum, on the other hand has a specific gravity of only 20%-25% of lead and approximately eight times the conductivity of lead. Obviously, from the standpoint of weight and conductivity, copper and aluminum are good candidates to replace lead as the substrate for electrodes. However both materials are very susceptible to corrosion in the presence of sulfuric acid, and cannot be used as the positive electrode in a lead acid battery if left unprotected. Either material can be used as the negative electrode, and copper has in the past. The conventional manner for plating lead is from an aqueous solution. When lead is plated from an aqueous solution, for one reason or another, the coatings are porous, and the sulfuric acid will quickly penetrate the coatings and attack the aluminum or copper. In such instances, and without any other treatment of the plated lead layer, the copper and aluminum plates have not survived the charging operation. As a result, aluminum based substrates with lead applied in such a manner have not succeeded in the past.
The present invention is directed toward reducing the weight per unit mass and improving the conductivity of the electrochemical cell by replacing the lead or lead alloy film therein with a lighter weight, low resistance, conductive material that is coated with or plated by lead or some other conductive coating. Further, the outer lead coating is accomplished in a manner that protects the non-lead film substrate from the corrosive effect of the electrolytic acid in the cell. Toward this end, the present invention is an electrode plate for a lead-acid electrochemical cell formed primarily of a lightweight foil or film substrate coated with a layer of conductive, but corrosive resistant material such as lead. The foil has a thickness of between about 0.005 inches and 0.030 inches. The use of a lightweight foil, such as aluminum, as a current collector substrate has numerous advantages compared to conventional lead or lead alloy current collectors. Specifically, a lighter current collector means a higher specific energy (WH/KG) for lead acid batteries. Since aluminum has a much higher conductivity, or lower resistivity, compared to lead or lead alloys, it facilitates fast formation and fast charging of the battery due to reduced ohmic losses. As a current collector, aluminum also delivers higher power compared to lead, an important factor in applications involving high current discharges. An aluminum foil current collector ensures higher utilization of the electrode material as a result of improved current distribution within the electrodes. Other, lightweight, relatively lower resistively foils such as copper, nickel, tin, silver, or magnesium may also be used as the substrate.
One embodiment of the present invention utilizes an aluminum foil substrate coated with an outer layer of corrosive resistant material such as lead or a lead alloy. To ensure that the outer conductive layer suitably protects the aluminum substrate from attack by the electrolytic acid, the outer layer may be sealed by any one of several conventional means including immersing the plate in a heated non-aqueous liquid such as peanut or canola oil at a temperature that causes any pores in the conductive outer layer to close and seal. While the aluminum substrate may be coated directly in the manner described herein, the lead coating should desirably be about 0.001-0.005 inches thick to provide corrosion protection for the substrate.
If desired, an intermediate corrosion prevention layer may be formed between the aluminum foil substrate and the lead outer coating to provide further corrosion protection for the underlying substrate. Where the intermediate layer is applied, the lead outer coating of the present invention may be reduced to about 0.001 inches to 0.002 inches. The added thin corrosion prevention layer further provides the advantage of providing a strong bonded structure, resulting in an extended battery life.
The continuous outer layer of lead or lead alloy is applied by dipping the coated foil in a molten lead or lead alloy, immersing the foil in a molten salt solution, electroplating, vacuum deposition, spray deposition, or plasma deposition. However, other plating techniques conventional in the art may also be used depending upon the degree of sealing desired in the outer conductive layer.
The electrode plates of the present invention may be subsequently cut into strips of desired width and length and coated with conventional negative or positive pastes to create corresponding negative or positive electrodes. These electrodes may be utilized in any number of battery cell configurations. Alternating positive and negative electrode plates are generally separated by a separator such as a thin glass mat and arranged in a conventional parallel plate arrangement. Since the electrodes are flexible and quite thin in construction, alternating positive and negative plates may also be separated by a thin glass separator and rolled or coiled in a cylindrical or oblong configuration. Alternatively, the thin plates may be fan-folded in an accordian fashion to create a cell configuration of desired thickness having enhanced structural stability.